The present invention relates to mobile communication systems, and more particularly to methods and apparatuses for serving user equipment in both the uplink and downlink directions in a mobile communication system.
In a mobile communication system, User Equipment (UE) is capable of maintaining its communication service while moving throughout a geographical coverage area of the system. To enable this capability, the system's coverage area is provided with a number of geographically separated base stations that serve as the UE's portal to the mobile communication system. The UE always sends data to, and receives data from, the mobile communication system via a base station. In a typical system, the UE is connected in both the uplink and downlink directions to the base station having the most favorable radio conditions. The area covered by the base station is usually called a cell, and the cell to which the UE is connected is usually referred to as a serving cell.
To achieve compatibility and interoperability between UE's made by various manufacturers, as well as to avoid causing disturbing interference to unrelated devices, mobile communication systems typically need to comply with various standards and government regulations. A number of these are used and well-known in the art. To facilitate this discussion, terminology and network configurations that comply with the Wideband Code Division Multiple Access/High-Speed Packet Access (WCDMA/HSPA) standard are used herein because these are known and will be readily understandable to the person of ordinary skill in the art. However, the use of this terminology and these configurations is done solely for the purpose of example rather than limitation. The various inventive aspects to be described in this document are equally applicable in many different mobile communications systems complying with different standards.
A simplified cell planning diagram is depicted in FIG. 1. A core network (not shown) is connected to a radio access network that includes one or more radio network controllers (RNCs) 101-1, 101-2, . . . ,101-N (generally referred to by reference numeral 101). Each RNC 101 is capable of communicating with every other RNC 101 in the same network. As can be seen in FIG. 1, one RNC 101 connects to one or more base stations, 103-1, 103-2, . . . , 103-M (generally referred to by the reference numeral 103). In a WCDMA/HSPA system, the base station functionality apart from the antenna(s) is denoted “NodeB”. The NodeB is a logical node handling the transmission and reception of a set of cells. Logically, the antennas of the cells belong to the NodeB but they are not necessarily located at the same antenna site. Thus, one NodeB can be responsible for one or more cells. It is the ability of serving cells not transmitted from the same antenna site that makes a NodeB different compared to what in other types of systems are called a “Base Transceiver Station (BTS)”, “Base Station (BS)”, or “Radio Base Station (RBS)”. However, in this specification the term “base station” is used as a generic term, rather than a system-specific term, to further emphasize that the invention is not limited to applications in only the specific exemplary systems.
The specific functionality of an RNC can vary from one system to another, but generally an RNC is responsible for controlling base stations with respect to mobility and radio resource management.
Because the RNC 101 can control several base stations 103, it can be responsible for a very large geographical area. Furthermore, the interface between the RNCs 101 makes it possible to have a coordinated approach in the whole coverage area of the network. However, only one RNC 101, denoted the “controlling RNC”, is the master of one base station 103. For example, in a WCDMA/HSPA system, the controlling RNC sets the frequencies that the NodeB will use in its cells. The controlling RNC also allocates power and schedules the common channels of the cells of the NodeB. It also configures what codes and maximum power levels will be used for the High-Speed-Downlink Shared CHannel (HS-DSCH). The controlling RNC is also the decider of whether a user is allowed to use the radio resources in a cell belonging to one of its NodeBs and in that case which radio resource.
It should be stressed that the above description of network component functionality (e.g., that of the RNCs 101) is meant to provide only an example. More generally, RNCs 101 should be considered to be network nodes having some intelligence that controls the base stations 103.
When configuring an actual embodiment of a mobile communication system, it is typical for different cells to have different sizes and shapes. This can be due to, for example, different terrain conditions such as buildings or mountains, and/or different transmit power capabilities of the different base stations 103. FIG. 2 is a block diagram illustrating an RNC 205 that controls a number of cells of varying sizes. This diagram illustrates the possibility not only of different size cells, but also of a small cell 201 being placed within a larger cell 203.
To ensure that a serving base station is able to receive transmissions from one of the UEs that it is serving, the UE must transmit its signal at a sufficiently high power level. If the channel conditions are poor and/or if the UE is far away from the serving base station, the UE must transmit at even higher power levels. This detrimentally affects the battery life of the UE. To extend the battery's life, it would be preferable for the UE to transmit at as low a power level as possible.
Another problem with a UE transmitting at excessively high power levels derives from the fact that a UE transmits in all directions. This increases the total interference in the communication system. Again, having the UE transmit at lower power levels would address this problem.
For the above and other reasons, it is desired to provide a way of serving a UE in a mobile communication system in a way that reduces the need for the UE to transmit at high power levels.